Space debris (ESA photo)

WASHINGTON — While launch providers are doing a better job at disposing of upper stages left behind in orbit, rocket bodies still constitute the most dangerous pieces of orbital debris.

The European Space Agency released Oct. 12 its annual Space Environment Report, the agency’s assessment of orbital debris. The report identifies more than 25,000 tracked objects, including satellites, upper stages and debris.

While collisions between objects can create additional debris, a bigger concern is the breakup of satellites or rocket bodies caused when batteries or propellant tanks on them explode. “The biggest contributor to the current space debris problem is explosions in orbit, caused by leftover energy — fuel and batteries — onboard spacecraft and rockets,” Holger Krag, head of ESA’s Space Safety Program, said in a statement. “Despite measures being in place for years to prevent this, we see no decline in the number of such events.”

Rocket bodies are of particular concern because their size can create a large number of objects. A breakup of a Japanese H-2A upper stage in 2019 created more than 70 pieces of tracked debris, one of which came close enough to the International Space Station in September to warrant a maneuver by the station.

The report found that launch operators are doing a better job of disposing of upper stages. In 2019, more than 70% of rocket bodies complied with orbital debris mitigation guidelines, compared to only about 20% in 2000.

However, many upper stages from launches decades ago remain in low Earth orbit and continue to pose a problem. In a presentation at the 71st International Astronautical Congress this week, Darren McKnight of Centauri presented an analysis of the 50 “statistically most concerning” debris objects in low Earth orbit.

That list was developed by combining 11 separate analyses by 19 authors, who used different approaches for ranking based on criteria such as the mass of the objects, the probability of collision and persistence of their orbits. Those analysis were combined into a single master list.

The top 20 objects in that master list are all a single class of upper stages known as SL-16, from the Zenit family of rockets. The stages are all large and are in similar orbits, raising the risk of collision, McKnight noted. The highest ranked satellite, ESA’s Envisat, which malfunctioned before it could be deorbited, is 21st on the list.

Overall, 78% of the objects on the list are rocket bodies, and 80% of the objects were launched before 2000, when countries started adopting orbital debris mitigation guidelines. “These are really lingering problems from early in the Space Age,” he said.

The development of the top-50 list can support future efforts to mitigate the risk through active debris removal. McKnight said that the risks of many of these objects are coupled to others on the list. “If you remove one, the other one may drop way down on the list,” he said. “We don’t have to take them all out. We can reduce their risk enough by taking one selectively out.”

There’s no single approach, he said, to addressing near-term space safety and long-term space sustainability concerns. “We need to look at not just a single solution but a series of solutions.”

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...